This invention relates to multiple component spunbond nonwoven fabrics and composite sheets thereof, that are soft, strong, and have improved thermal bonding properties.
Sheath-core staple fibers that comprise a sheath polymer having a lower melting point than the core polymer are known in the art for use as binder fibers. Binder fibers are staple fibers that can be used alone or in blends with other staple fibers to form a nonwoven web that can be bonded by heating to a temperature that is sufficient to activate the binder fibers, causing the surface of the binder fibers to adhere to adjacent fibers. Ahn et al. EPO Published Application No. 0366379 describes sheath/core binder fibers having a polyester core and a sheath consisting essentially of an organic copolymer of ethylene and a polar co-monomer. Kim et al. Korean Patent No. 9104459 describes sheath-core staple fibers suitable for use as binder fibers wherein the sheath is prepared by adding 0.1-10 weight percent of an ionomer to high density polyethylene or normal chain low density polyethylene. Kubo et al. U.S. Pat. No. 5,277,974 describes heat bondable binder fibers comprising a sheath component formed of ethylene co-polymerized with at least one component selected from the group consisting of an unsaturated carboxylic acid, a derivative thereof, and an unsaturated carboxylic anhydride in an amount of 0.1-5 mole percent, and a core component formed from a fiber-forming polymer having a melting point at least 30° C. higher than the copolymer sheath. In certain end uses, for example medical garments, such staple-based products may not have sufficient surface stability, resulting in release of fibers from the fabric during use (linting). The strength of such materials may also be lower than desired.
Spunbond nonwovens formed from sheath-core fibers that comprise a sheath polymer that melts at a lower temperature than the core polymer are also known. For example, Terakawa et al. U.S. Pat. No. 6,187,699 describes multi-layer nonwoven fabrics that comprise a composite spunbond nonwoven fabric composed of multiple component spunbond fibers formed from a low melting point resin and a high melting point resin wherein the low-melting resin component forms at least a part of the spunbond fiber surface. The spunbond fibers can be spun in a sheath-core configuration, side-by-side, or multi-layer configuration. The spunbond nonwoven is described as a partial thermal fusion product of the multiple component spunbond fibers by the mediation of the low melting point resin.
Multi-layer nonwoven laminates comprising spunbond and melt-blown layers, such as spunbond-meltblown-spunbond (“SMS”) nonwovens are also known in the art. The exterior layers of a SMS nonwoven fabric are spunbond nonwoven webs that contribute strength to the overall composite, while the middle or core layer comprises a meltblown web that provides barrier properties. Similarly, composite nonwovens comprising additional layers of spunbond or meltblown webs can be prepared, as in spunbond-meltblown-meltblown-spun bond (“SM MS”) nonwovens.
It is also known to form thermally-bonded nonwoven fabrics that comprise fibers made from blends of a lower melting polymer and a higher melting polymer. Gessner U.S. Pat. No. 5,294,482 describes a thermally-bonded nonwoven fabric comprising multiconstituent fibers composed of a highly dispersed blend of at least two different immiscible thermoplastic polymers that has a dominant continuous polymer phase and at least one non-continuous phase dispersed therein. The polymer of the non-continuous phase has a polymer melt temperature at least 30° C. below the polymer melt temperature of the continuous phase and the fiber is configured such that the non-continuous phase occupies a substantial portion of the fiber surface. Harrington U.S. Pat. No. 6,458,726 describes thermally bonded nonwoven fabrics comprising skin-core fibers wherein the fibers are composed of a polymer blend of a polyolefin and a polymeric bond curve enhancing agent, such as ethylene vinyl acetate polymers. The polyolefin is preferably polypropylene and the skin layer is produced by oxidation, degradation and/or lowering of the molecular weight of the polymer blend at the surface of the fiber compared to the polymer blend in an inner core of the fiber. Thus, the skin-core structure comprises a modification of a blend of polymers to obtain the skin-core structure and does not comprise separate components being joined along an axially extending interface, such as in sheath-core and side-by-side bicomponent fibers.
For certain nonwoven end uses, it is desirable that the nonwoven fabric have good heat-sealing properties when thermally bonded to an identical nonwoven fabric layer or to a dissimilar layer such as a nonwoven fabric comprising fibers of a different polymer composition. For example in protective apparel uses such as medical garments, it may be desirable to prepare the garments by heat-sealing the seams to avoid formation of holes that occurs when needles are inserted during stitching. Alternately, reinforcing pieces may be thermally bonded in place instead of using an adhesive or stitching process. In addition to good heat-sealing properties, it is desirable that the nonwoven fabrics have high strength while also being as soft and drapeable as possible. For medical end uses, it is also desirable that the nonwoven fabrics be made of fibers of polymers that can be sterilized with gamma radiation. SMS fabrics have traditionally been polypropylene-based and have the limitation that they cannot be sterilized with gamma radiation because the fabrics are discolored and weakened as a result of the sterilization process. In addition, gamma-irradiation of polypropylene based fabrics results in the generation of unpleasant odors. This presents a significant problem for polypropylene-based SMS fabrics because radiation sterilization is commonly used throughout the medical industry.
There remains a need for low-cost nonwoven fabrics that have an improved combination of strength, softness, and heat sealability and that can be sterilized by gamma radiation without significantly degrading the properties of the fabric and/or generating unpleasant odors.